Here are some interesting current statistics from the Kinsey Institute:
http://www.iub.edu/~kinsey/resources/FAQ.html

Here’s some great CDC data on adolescent sexuality statistics, but the data only goes back to 1991:
http://apps.nccd.cdc.gov/youthonline/App/QuestionsOrLocations.aspx?CategoryID=1508

And, of course, the Guttmacher Institute has a huge store of data and statistics:
http://www.guttmacher.org/index.html

The Guttmacher Data Center is a particularly easy way to get some information:
http://www.guttmacher.org/datacenter/index.jsp

Now, all of this great data aside, I am looking for one specific place to send you for the historical data - so stay tuned!